In this post, I will try to compare and contrast Julia, R, and Python via a simple maximum likelihood optimization problem whose motivation comes from the credit risk domain and is discussed in more detail inthis post
Side note  the author's experience level at the time of writing
Language  Level of experience 

R  9 years 
Julia  6 months 
Python  beginner 
For such a simple optimization problem, R, Julia, and Python/SciPy will all do a competent job , so there is no clear winner. However as noted by Julia discourse member ChrisRackauckas
if you wish to solve only a single optimization problem and that problem takes <10 seconds, then Julia’s long initial compilation is something you want to avoid. For long enough problems, or for solving multiple optimization problems, the compilation time is not noticeable.
The optimization problem
Given observations
Q
1
,
Q
2
,
.
.
.
,
Q
n
Q_1,, Q_2,, ...,, Q_n
Q 1 , Q 2 , . . . , Q n , we aim to find paramters
μ
mu
and
σ
sigma
that optimize this likelihood function
L
=
∏
(
ϕ
(
Q
i
,
μ
,
σ
)
Φ
(
max
Q
t
,
μ
,
σ
)
)
L = prodleft(frac{phi(Q_i,mu,sigma)}{Phi(max Q_t,mu,sigma)}right)
L = ∏ ( Φ ( max Q t , μ , σ ) ϕ ( Q i , μ , σ ) )
often we try to optimize the loglikelihood instead
log
L
=
l
=
(
∑
i
ϕ
(
Q
i
,
μ
,
σ
)
)
−
n
Φ
(
max
Q
t
,
μ
,
σ
)
log L = l = left(sum_i phi(Q_i,mu,sigma)right)  nPhi(max Q_t,mu,sigma)
lo g L = l = ( i ∑ ϕ ( Q i , μ , σ ) ) − n Φ ( max Q t , μ , σ )
side note  a more efficient numerical solution exists 

This Ergashev et al., 2016 paper (behind a paywall) derived a necessary condition for when a solution exists and derived an equation in one parameter which when solved (using simple uniroot techniques) can recover both
μ mu and σ sigma . From my testing, applying Ergashev's formula yields about 50x speed up to the R solution. However, in this blogpost, I aim to compare and contrast the optimization function in Julia vs. R vs. Python and hence I have chosen not to implement Ergashev's methods. 
Julia solution
Below is my Julia implementation using Optim.jl
In Julia, one can use symbols in variable names, so I have used
μ
σ
musigma
as a variable name. Julia also has a popular package called JuMP.jl for optimization problems. However, the Optim.jl package is more than adequate for such a simple problem, and I will only look at JuMP.jl in the future when dealing with more advanced optimization problems.
I have noted the time it takes to perform the first run of the optimization problem is 7.5 seconds. This is considerably slower than both R and Python. In regards to this, Julia Discourse member ChrisRackauckas have pointed out
If you want to solve a problem that takes 3000 seconds to solve, the first time in a Julia session will make it take 3007 seconds making the total runtime more relevant to the total time than compilation. If you want to solve many 100 10 second optimization problems, the first will take 17 seconds, and subsequent calls will not have the compilation and will take roughly 10 seconds, making the total runtime 1007 seconds and thus the individual problem each is more relevant than the compilation time. If you want to solve a single 5second optimization problem, it’ll be 12 seconds. Thus the compilation time issue can be annoying for interactive use when you want to solve a single problem but doesn’t effect performancesensitive uses.
He also notes that
Julia v0.7 does have an interpreter though which promises to reduce compilation time in exchange for less optimizations and this may be an interesting middle ground for the specific interactive use case. Additionally, tools like PackageCompiler.jl may become more common, limiting the “startup time” of package code that is noticed here.
In the below, I have hardcoded the values of Q_t
that I would like to use in the MLE estimation.
using Distributions, Optim # hard coded dataobservations odr=[0.10,0.20,0.15,0.22,0.15,0.10,0.08,0.09,0.12] Q_t = quantile.(Normal(0,1), odr) # return a function that accepts `[mu, sigma]` as parameter function neglik_tn(Q_t) maxx = maximum(Q_t) f(μσ) = sum(logpdf.(Truncated(Normal(μσ[1],μσ[2]), Inf, maxx), Q_t)) f end neglikfn = neglik_tn(Q_t) # optimize! # start searching @time res = optimize(neglikfn, [mean(Q_t), std(Q_t)]) # 7.5 seconds @time res = optimize(neglikfn, [mean(Q_t), std(Q_t)]) # 0.000137 seconds # the mu and sigma estimates Optim.minimizer(res) # [1.0733250637041452,0.2537450497038758] # or # use `fieldnames(res)` to see the list of field names that can be referenced via . (dot) res.minimizer # [1.0733250637041452,0.2537450497038758]
which gave me the following output which is by far the most descriptive out of the Julia/R/Pythonverse.
Results of Optimization Algorithm * Algorithm: NelderMead * Starting Point: [1.1300664159893685,0.22269345618402703] * Minimizer: [1.0733250637041452,0.2537450497038758] * Minimum: 1.893080e+00 * Iterations: 28 * Convergence: true * √(Σ(yᵢȳ)²)/n < 1.0e08: true * Reached Maximum Number of Iterations: false * Objective Calls: 59
What was nice about Julia?
Rating  Description 

Truncated(DN, lower, upper) is a wonderfully simple way to define truncated distribution 

A logpdf function that works for any distribution 

Very clear and informative output 
What was not so nice about Julia?
R solution
R has a truncnorm
package for dealing with truncated normals.
odr=c(0.10,0.20,0.15,0.22,0.15,0.10,0.08,0.09,0.12) x = qnorm(odr) library(truncnorm) neglik_tn = function(x) { maxx = max(x) resfn = function(musigma) { sum(log(dtruncnorm(x, a = Inf, b= maxx, musigma[1], musigma[2]))) } resfn } neglikfn = neglik_tn(x) system.time(res < optim(c(mean(x), sd(x)), neglikfn)) res
which gave the output
$par [1] 1.4294776 0.3555692 $value [1] 3.020876 $counts function gradient 57 NA $convergence [1] 0 $message NULL
What was nice about R?
Rating  Description 

Has a package for truncated normal  
Gives me results right away; this wouldn't be such a plus if it weren't for Julia's slow compilation speed 
What was not so nice about R?
Rating  Description 

A couple of minor things: no log density for truncated normal; and no easy way to define truncated distribution for arbitrary distributions; sparse output (print) 
Python solution
Even though I have no experience with Python, simple Google searches allowed me to come up with this solution. I have used the Anaconda distribution which saved me a lot of hassle in terms installing packages, as all the packages I need are preinstalled.
import numpy as np from scipy.optimize import minimize from scipy.stats import norm # generate the data odr=[0.10,0.20,0.15,0.22,0.15,0.10,0.08,0.09,0.12] Q_t = norm.ppf(odr) maxQ_t = max(Q_t) # define a function that will return a return to optimize based on the input data def neglik_trunc_tn(Q_t): maxQ_t = max(Q_t) def neglik_trunc_fn(musigma): return sum(norm.logpdf(Q_t, musigma[0], musigma[1])) + len(Q_t)*norm.logcdf(maxQ_t, musigma[0], musigma[1]) return neglik_trunc_fn # the likelihood function to optimize neglik = neglik_trunc_tn(Q_t) # optimize! res = minimize(neglik, [np.mean(Q_t), np.std(Q_t)]) res
and these are the results
fun: 1.8930804441641282 hess_inv: array([[ 0.01759589, 0.00818596], [ 0.00818596, 0.00937868]]) jac: array([ 3.87430191e07, 3.33786011e06]) message: 'Optimization terminated successfully.' nfev: 40 nit: 6 njev: 10 status: 0 success: True x: array([1.07334252, 0.25373624])
What was nice about Python?
Rating  Description 

Easy to google even for beginners  
Gives me results right away 
What was not so nice about Python?
Rating  Description 

The output could be nicer, but a minor point 
Conclusion
For such a simple optimization problem, you can't go wrong choosing any of the three languages/ecosystems. All three languages/ecosystems allow you to define closures which I find to be a good way to generate functions that embed the data that it needs, leaving you with just a function with the distribution parameters as the only arguments.
I would not choose to use Julia if I only need to run simple (sub10 seconds) optimizations that I only intend to run once or twice due to long compilation times. If your problem is fairly standard you can get around this by using PackageCompiler.jl
to precompile the functions; but that feels like an overcomplication for such a simple task that both R and Python/SciPy can handle easily.